1. Field of the Invention
The present invention relates to a control unit to control a cycloconverter which is used in variable speed operation of an AC motor such as a synchronous motor, an induction motor or the like, and performs frequency conversion of an AC power source using, for example, a thyristor element to perform commutation by the AC power source. More particularly, the invention relates to a control unit wherein a continuous cycloconverter of the non-circulating current type which is the most suitable to drive an induction motor a is operated accurately with very small phase error and very small amplitude error in three phase current control.
2. Description of the Prior Art
According to development of power electronics and social needs in recent years, rotation drive devices in various industrial fields, for example, a control system for an AC motor such as an induction motor are significantly varied. In other words, because of rapid development in thyristor elements and their control circuit technology, application of a cycloconverter to a driving power source of an AC motor or the like is becoming active. The cycloconverter is a static frequency conversion device where two rectifier circuits of the same sort (called positive and negative converters for convenience) are connected in anti-parallel connection and alternately operated by an AC power source so as to obtain alternating voltage with a frequency different from that of the power source.
FIG. 1 is a block constitution diagram illustrating a control unit for a cycloconverter of the non-circulating current type in the prior art disclosed, for example, an article "AC variable speed drive by a vector control cycloconverter" in OHM Magazine, April 1981. In FIG. 1, numeral 1 designates a speed setting device, numeral 2 designates a speed controller, numeral 3 designates a magnetic flux command operator, numeral 4 designates a magnetic flux controller, numeral 5 designates a magnetic flux operator, numeral 6 designates a divider, numerals 7, 8 designate vector rotation devices, numeral 9 designates a two-phase/three-phase conversion circuit, numeral 10 designates an armature current controller, numeral 11 designates a gate controller for armature current, numeral 12 designates an AC power source, numeral 13 designates a sinusoidal cycloconverter of the three-phase non-circulating current type (hereinafter referred to as "three-phase cycloconverter"), numeral 14 designates a synchronous motor, numeral 15 designates a rotor position detector, numeral 16 designates a tacho generator, numeral 17 designates a field current controller, numeral 18 designates a gate controller for field current, numeral 19 designates a field coil, and numeral 20 designates a switching thyristor of the coil 9. FIG. 2 is a circuit constitution diagram illustrating a current control circuit of one phase. In FIG. 2, numeral 21 designates a current transformer, and numeral 22 designates a load including the synchronous motor 14.
Next, the operation will be described. Torque set value T.sub.e * is supplied by the speed controller 2 and divided by magnetic flux .PHI. in the divider 6 so as to obtain torque current set value i.sub.T *. In this case, magnetizing current set value i.sub.m * is made zero for the operation in motor power factor cos.phi..sub.M =1. The torque current set value i.sub.T * and the magnetizing current set value i.sub.M * are quantities viewed from the rotating magnetic flux axis and therefore DC quantities. In order to convert these set values into actual armature current (AC quantity), these values are first converted into two-phase alternating currents i.sub..alpha. *, i.sub..beta. * by the vector rotation device 7 using the magnetic flux directional signals cos.phi. , sin.phi. as parameters obtained from the rotor position detector 15, the vector rotation device 8 and the magnetic flux operator 5. The converted two-phase alternating currents i.sub.60 *, i.sub..beta. * are further converted into three-phase alternating current set values i.sub.R * , i.sub.S * , i.sub.T * by the two-phase/three-phase conversion circuit 9. The three-phase cycloconverter 13 has a current control loop per each phase using the alternating currents i.sub.R *, i.sub.S *, i.sub.T * as set values, and is operated by the current controller 10 and the gate controller 11 for armature current so that the output currents i.sub.R, i.sub.S, i.sub.T of the three-phase cycloconverter 13 coincide with the set values i.sub.R * , i.sub.S *, i.sub.T *.
Thus the three-phase cycloconverter 13 is operated by the current control loop as shown in FIG. 1. This current control loop will be described by a current control circuit of one phase shown in FIG. 2. Current command of AC quantity, being the alternating current set value i.sub.R *, is given and compared with the output current i.sub.R flowing through the load 21, and the deviation .epsilon. is amplified by the current controller 10 and then supplied as the voltage command V.sub.R * to the three-phase cycloconverter 13. In such control manner, the load current i.sub.R coinciding with the alternating current set value i.sub.R * as the command value is intended to flow.
Since the control unit for the cycloconverter of non-circulating current type in the prior art is constituted as above described, when the output frequency becomes higher due to the delay element of the current control circuit, an amplitude error and a phase error will be produced between the alternating current set value i.sub.R * and the load current i.sub.R as shown in FIG. 3.